An experimental and theoretical study is being undertaken of the magnetic spin-spin interactions which arise when the cobalt to carbon bond is cleaved in coenzyme B12 or model compounds for B12. We will collect experimental EPR data on a new EPR set-up now under construction. Our objectives are to elucidate the earliest chemical steps that follow cleavage of this bond, and compare our results with a hypothesis on the physical meaning of "spin-spin geometry". The exchange couply data are also to be compared to a recent proposal (R.E.C.) on a limit-function to superexchange.